Resistance to apoptosis contributes to tumorigenesis as well as poor therapeutic outcome. The tumor suppressor p53 mediates two predominant cellular responses, cell cycle arrest and apoptosis, but the molecular basis for this cell fate determination has remained elusive. It is proposed that an interplay between p53-independent effectors and p53-dependent gene expression may be the crucial determinant for cellular outcome. The proposed studies test this hypothesis by examining the interplay between Sp/KLF family members and p53 on regulating gene expression, focusing on the role of p21CIP1 as an attenuator of the apoptotic response, and elucidating whether alterations in basal levels of expression of key members of apoptotic pathways contribute to cell fate decisions. Multiple p53 response elements located in both the promoter and first intron are involved in the DNA damage-induced upregulation of p21CIP1 with the two sites in the p21CIP1 promoter being regulated in distinct manners. In the first aim, the molecular basis for the differences between the two types of p53 sites will be elucidated and the interplay between p53, Sp1, and other Sp/KLF family members will be examined. The key role of other cellular factors such as Sp/KLF family members in p53-dependent gene regulation will be addressed as a means for influencing cellular outcomes. The ability of the cyclin-dependent kinase inhibitor p21CIP1 to interfere with apoptosis and influence cell fate has been suggested by published studies as well as preliminary data. In the second aim, the role of p21CIP1 in attenuating the cell death response will be validated and characterized and the underlying molecular basis for this intriguing effect of p21CIP1 will be elucidated. Regulating the basal levels of expression of key components of apoptotic pathways is an alternative mechanism for determining cell fate outcomes. Elements in the bax gene that confer constitutive transcriptional regulation of Bax have been identified. NHE1 is a novel p53 target gene that has been shown to regulate the anti-apoptotic effects of Bcl-XL via deamidation. In the third aim, the molecular interplay between Bax, NHE1, and Bcl-XL will be explored to determine whether such mechanisms can explain the apoptotic resistant phenotype of particular tumor cells. Depending upon particular cellular conditions, the tumor suppressor protein p53 induces growth arrest or mediates an apoptotic response. The optimal therapeutic response to DNA damage caused by many chemotherapeutic agents is cell death rather than inhibition of cell cycle progression. Elucidating the molecular mechanisms that are responsible for regulating the ability of p53 to trigger apoptosis versus arrest may lead to more effective therapeutic intervention and a way to overcome the chemotherapeutic-resistant phenotype found in many tumors.